When is a thin battery a useful battery?

As technology moves forward, devices are tending to get smaller and lighter. So, at face value batteries need to get thinner right? Then we can make mobile phones even thinner? Let me explain why this is not necessarily true.

The amount of energy stored in a battery is directly proportional to the volume of material in the battery. More material volume = more energy stored, so more talk time on your phone, more range on your car etc. If we make the battery thinner in the Z dimension, then the area covered by that battery in X and Y needs to increase to maintain the volume and therefore the amount of energy stored. In fact, the energy storage efficiency often gets worse as the battery gets thinner, so the area will need to go up even more.

The only way we’re going to make our device smaller is if we can increase the energy density of the battery. Energy density is the amount of energy that can be stored per unit of volume and is mainly defined by the Chemistry of the battery and the chemical reactions that are occurring during charge and discharge. According to Wikipedia Lithium Polymer (LiPo) batteries have an energy density of 250 to 730 Watt-hours per litre (Wh/L). These units of measurement are a bit abstract, so to look at a real example, the battery from a Samsung S3 Gear is 22.5 mm x 23.3 mm x 5 mm (2.62 cm³) and stores 1.47 Watt Hours (i.e. it can provide 380 mA at a nominal voltage of 3.85 V for 1 hour). This is 561 Wh/L and means 1 cm³ of volume stores 0.56 Wh of energy. If you want to store 1 Wh of energy then it will either need to be 20 mm x 20 mm x 4.5mm or 95 mm x 95 mm x 0.2 mm. The thickness of the battery makes a significant difference to the area covered!

Where next?

Currently we’re in a period of optimisation. The LiPo chemistry has been the chemistry of choice since the early 2000s and is well established. Battery manufacturers are optimising their cell designs to allow them to charge to a higher voltage and thus store slightly more energy (LiPos were once charged to 4.2V, but now consumer devices have batteries that are charged to 4.4V). The question is, what will be the next step change be, like we saw when we went from the Lead Acid chemistry (60 Wh/L) to NiMH chemistry (400 Wh/L)? This will really enable batteries, and therefore devices, to get smaller. At the moment there are some lithium chemistry variations (Lithium Titanate, Lithium Ceramic, Lithium-Iron-Phosphate) appearing on the market that have some technical advantages, but they have not become main stream (yet), and they are not really step changes in Energy Density. There are also some "vapour-ware" batteries around. For example, I’ve seen one battery manufacturer claim they plan to achieve 1000 Wh/L for their thin battery, when they are currently only achieving 5 Wh/L in real life!

So, coming back to thin batteries, the only applications where I think current thin battery technology would be beneficial is if you want to make a thin thing, and are not so worried about surface area. e.g. putting some electronics in a credit card.

How do you change the rules?

Thin batteries, or in fact any battery, would become a game changer if:

The battery can be embedded inside a circuit board to power a small, low power device where the size and bulk of the battery connector is significant compared to the amount of energy storage required. Batteries embedded like this are slowly starting to become real, however the applications are limited at the moment due to the cost and the small amount of energy that can be stored (a few tens of mAh).

The battery could be distributed – imagine an electric bike with a frame “coated” in battery. A few millimetres extra diameter on all the frame tubes could remove the need for a large bulky centralised battery.

The battery could have two functions - be part of the device structure and store energy. For example, a watch wrist strap that also stores energy and powers the watch or house bricks that can also store excess energy generated by roof top solar panels.

The issue, particularly with those last two concepts, is if the battery becomes more and more integrated with the device, then battery lifetime becomes more critical as battery replacement becomes impossible. We already generate enough waste and I’d hate us to start throwing away a bike frame or a house every couple of years as the battery got worn out! However, long life rechargeable batteries are a possibility. I’ve worked on a project where we were using a new Lithium Titanate chemistry which was specified for 10,000 cycles – that is about three charge cycles per day for 10 years!

Please note that I have made some simplifications in this article. If you’d like to talk about batteries in more detail, please get in touch with us.

Gary is an Electronics and Software Engineer by training and specialises in Electro-Mechanical Systems Engineering. He has worked on some of the largest and most complex multi-disciplinary projects at Cambridge Consultants.